I once argued (by correspondence) with an agnostic about this. Six months we traded letters. I pulled out all the usual stuff: philosophy, bibliographic evidence, logic etc. etc., all to no avail. Of course his bottom line claim was that I had simply not given him enough evidence, and the last I heard of him, he told me he was investigating zen buddhism.
I have learned the hard way that objective arguments are useful for keeping a skeptic from changing the subject, but that is about all. The problem with evidential argument is that the doubter will keep raising the bar high enough that has an excuse not to believe. In essence, the agnostic has a “fundamentalist agenda” of his own, one which allows him to live his life as he jolly well pleases. A hard, tedious lesson to learn—but I learned it. Bill O’Reilly likes to use the term “no spin,” but (contrary to what Bill O’Reilly claims) everyone has a spin. It’s a question of which bias is the best bias to be biased with.
The best bias is the one the best explains the observed phenomena. One reason I am a conservative.
The reason I am a Christian is the Grace of God, pure and simple. It also helps that it comports with the facts that I have observed.